Decay Of Excited Electrons Research Articles

Impact of ultrafast transport on the high-energy states of a photoexcited topological insulator

Ultrafast dynamics in three-dimensional topological insulators (TIs) opens new routes for increasing the speed of information transport up to frequencies thousand times faster than in modern electronics. However, up to date, disentangling the exact contributions from bulk and surface transport to the subpicosecond dynamics of these materials remains a difficult challenge. Here, using time- and angle-resolved photoemission, we demonstrate that driving a TI from the bulk-conducting into the bulk-insulating transport regime allows to selectively switch on and off the emergent channels of ultrafast transport between the surface and the bulk. We thus establish that ultrafast transport is one of the main driving mechanisms responsible for the decay of excited electrons in prototypical TIs following laser excitation. We further show how ultrafast transport strongly affects the thermalization and scattering dynamics of the excited states up to high energies above the Fermi level. In particular, we observe how inhibiting the transport channels leads to a thermalization bottleneck that substantially slows down electron-hole recombination via electron-electron scatterings. Our results pave the way for exploiting ultrafast transport to control thermalization time scales in TI-based optoelectronic applications, and expand the capabilities of TIs as intrinsic solar cells.

Anomalous ultrafast dynamics of hot plasmonic electrons in nanostructures with hot spots.

The interaction of light and matter in metallic nanosystems is mediated by the collective oscillation of surface electrons, called plasmons. After excitation, plasmons are absorbed by the metal electrons through inter- and intraband transitions, creating a highly non-thermal distribution of electrons. The electron population then decays through electron-electron interactions, creating a hot electron distribution within a few hundred femtoseconds, followed by a further relaxation via electron-phonon scattering on the timescale of a few picoseconds. In the spectral domain, hot plasmonic electrons induce changes to the plasmonic resonance of the nanostructure by modifying the dielectric constant of the metal. Here, we report on the observation of anomalously strong changes to the ultrafast temporal and spectral responses of these excited hot plasmonic electrons in hybrid metal/oxide nanostructures as a result of varying the geometry and composition of the nanostructure and the excitation wavelength. In particular, we show a large ultrafast, pulsewidth-limited contribution to the excited electron decay signal in hybrid nanostructures containing hot spots. The intensity of this contribution correlates with the efficiency of the generation of highly excited surface electrons. Using theoretical models, we attribute this effect to the generation of hot plasmonic electrons from hot spots. We then develop general principles to enhance the generation of energetic electrons through specifically designed plasmonic nanostructures that could be used in applications where hot electron generation is beneficial, such as in solar photocatalysis, photodetectors and nonlinear devices.

Photopotential decay delay on TiO2 surface modified with p‐benzaldehydes: consequences and applications

TiO2 (Anatase) surface has been modified with p‐substituted benzaldehydes (p = OCH3, CH3, H, CN, and NO2) and 4‐stilbene carboxaldehyde. Fourier transform infrared spectroscopy attenuated total reflectance spectroscopy, UV–Vis reflectance spectra, and theoretical calculations indicate that the TiO2 surface has been chemically modified and supported acetal formation by means of TiO2–OH reaction with the aldehyde. A steady state photocurrent was obtained during simulated UV light irradiation of the acetal‐TiO2 in aqueous solution. Once the light irradiation is turned off, open‐circuit potential decay measurements were used in order to determine the electron life‐time (tn). Excited electron decay is inhibited down to 1 s when the electron withdrawing delocalization capacity of the modified TiO2 increases. Electron life‐time also depends on the solution reduction capacity. However, the unmodified TiO2 life‐time does not. The TiO2 modification results in a new series of photocatalysts that improve the organic contaminants degradation in solution because slow electron decay also induces retardation of the electron‐hole recombination. Therefore, there is a linear relationship between the electron decay life‐time and degradation rate constant. However, when electron delocalization is further increased in a way that the electron life‐time becomes ca. 7 s, degradation rate is kept constant. Therefore, the extra electron stability compromise degradation in such a way that the modified nanoparticle switches from a useful oxidant agent to a material that favors the charge carriers separation through a stable radical anion formation. Copyright © 2014 John Wiley & Sons, Ltd.

Photoluminescence from Gold and Silver Nanoparticles

This review is an attempt to highlight some of the significant results of the work carried out on the photoluminescence from nanoparticles of the noble metals, particularly gold and silver, over the past two decades. Although quite an immense amount of reports can be found, those that have contributed in throwing some light on the underlying mechanism behind photoluminescence have been considered here. Interband radiative recombination of electrons in metals or photoluminescence (PL), though very weak, was first reported in Au, Cu and Au-Cu alloys. A simple model attributes the PL to the radiative recombination of conduction band electrons below the Fermi energy with d-band holes. Most of the mechanisms are based on this concept. Only small sized clusters are known to exhibit luminescence, with the appearance of additional features which changed with the surfactants suggesting ligand to metal charge transfer. Further, the observation that more polar ligands do indeed enhance the luminescence intensity supports ligand to metal charge transfer. A non-radiative decay of excited electrons from 6sp-band to interface electron energy levels or bands (IEEB), that could be created due to charge transfer from the ligand to the metal core, followed by radiative recombination of electrons from these levels with the hole in the d-band could be another possible mechanism, which is supported by the size independence of the PL emission peak position. However, it is possible that these mechanisms operate independently or even simultaneously depending on various factors like size, ligands, dispersion medium, particle surface topography and so on.

Enhanced Emission and Analyte Sensing by Cinchonine Iridium(III) Cyclometalated Complexes Bearing Bent Diphosphine Chelators

Ir(III) complexes of cyclometalating ligands derived from the natural product cinchonine and bent (4,6-bis(diphenylphosphino)phenoxazine (Nixantphos), 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos)) and planar diphosphine ligands (1,2-bis(diphenylphosphino)benzene (dppb)) exhibit good luminescence with quantum efficiencies higher than those of their parent congeners. Steric hindrance by both the bulky cinchonine-derived ligand and bent diphosphine could limit nonradiative energy transfer. The cinchonine-derived and parent complexes cover a broad emission range from 472 to 569 nm with quantum efficiencies up to 0.38 and lifetimes from 0.01 to 0.46 μs in degassed CH2Cl2 solution at room temperature. DFT calculations on selected examples are in good agreement with solid-state structures determined crystallographically and accurately predict wavelengths of emission by excited electron decay from a quinoline-centered orbital to an Ir 5d–phenyl molecular orbital. The complex [(pcn)2Ir(Nixantphos)][PF6] (2; pcn = 2′-phenyl-9-O-benzyl-10,11-dihydrocinchonine-C2,N) exhibits the highest quantum yield and could detect electron-deficient aromatic species at ppm levels.

First Study of Intersubband Absorption in Electrons on Helium under Quantizing Magnetic Fields

We present the first measurements of inter-subband absorption of microwaves in surface electrons on liquid helium subjected to perpendicular magnetic field \(B\). In quantizing \(B\), the power absorption shows intermittent regions of enhanced and suppressed absorption. This behavior is caused by strong variation of the excited-electron decay rate with \(B\). Particularly, fast decay due to elastic scattering provides condition for strong absorption and overheating of the electron system, while slow decay due to inelastic scattering limits absorption and causes its saturation. An unexpected feature is the strong suppression of absorption at magnetic fields where the inter-subband energy splitting is a multiple number of the cyclotron energy.

Search for excited leptons in pp collisions at [formula omitted

Results are presented of a search for compositeness in electrons and muons using a data sample of pp collisions at a center-of-mass energy s=7 TeV collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 5.0 fb−1. Excited leptons (ℓ⁎) are assumed to be produced via contact interactions in conjunction with a standard model lepton and to decay via ℓ⁎→ℓγ, yielding a final state with two energetic leptons and a photon. The number of events observed in data is consistent with that expected from the standard model. The 95% confidence upper limits for the cross section for the production and decay of excited electrons (muons), with masses ranging from 0.6 to 2 TeV, are 1.48 to 1.24 fb (1.31 to 1.11 fb). Excited leptons with masses below 1.9 TeV are excluded for the case where the contact interaction scale equals the excited lepton mass. The limits on the cross sections are the most stringent ones published to date.

ONE- AND TWO-PARTICLE PHENOMENA IN THE ELECTRONIC LIFETIMES IN METALS: QUASIPARTICLES AND TRANSIENT EXCITONS

The experimental and theoretical investigation of the lifetime of excited electrons is of great importance for a variety of different fields in condensed matter physics. In this review two distinct classes of processes are discussed, which determine the lifetime of excited electrons in crystalline systems. One class is single-particle processes, which in many cases is able to describe the decay of excited electrons. For systems with weakly correlated electrons the state-of-the-art method is the solution of the Dyson equation using the GW approximation for the electronic self-energy. If applicable this approach leads to very good results. However, many of the experimental studies about the lifetime of excited electrons have been done using time-resolved two-photon photoemission spectroscopy utilizing ultrashort laser pulses. This technique, applied to materials with localized d electrons, can lead to the creation of bound, excitonic-like states in metals on a very short time scale, which are beyond the physics described in the single-particle approach. In this review the experimental evidence for both mechanisms is given and the theoretical tools to describe them are discussed. Furthermore, theoretical results are presented and compared to experimentally available data.

Production and decay of excited electrons at the LHC

We study single production of excited electrons at the CERN LHC through contact interactions of fermions. Subsequent decays of excited electrons to ordinary electrons and light fermions via gauge and contact interactions are examined. The mass range accessible with the ATLAS detector is obtained.

Hole Dynamics in Noble Metals

Hole dynamics in noble metals (Cu and Au) is investigated by means of first-principles many-body calculations. While holes in a free-electron gas are known to live shorter than electrons with the same excitation energy, our results indicate that d holes in noble metals exhibit longer inelastic lifetimes than excited sp electrons, in agreement with experiment. The density of states available for d-hole decay is larger than that for the decay of excited electrons; however, the small overlap between d and sp states below the Fermi level increases the d-hole lifetime. The impact of d-hole dynamics on electron-hole correlation effects is also addressed.